There are pluses and minuses to both. When I was growing up, my dad owned a full-size motorhome and I had always said that I would never own a pull trailer camper because I loved the ability to be able to walk from the passenger seat back to the kitchen, bathroom, or bedroom and we could tow a small jeep so we had wheels to zip around town when we had hooked up the camper. Now that I have a Toyotahome, I technically could squeeze from the front seat to the back, but it would be easier to pull over, get out through the cab door, walk outside, and enter through the coach door. That, plus the fact that my Toy can't tow a car, means that I'm stuck driving the RV as my primary means of conveyance OR my wife has to follow me in her car. That isn't too big of an inconvenience for short, weekend trips, but might be a hassle on longer trips or long duration stays. Plus, it is inconvenient to have to unhook my electric, water, sewer, and cable TV lines every time I want to drive into town to grab something to eat. If I was going to be living in my camper, I would prefer to get a bigger motorhome or get a truck and pull trailer.
I love my little Dolphin, and hats off to you guys who are able to do really long duration stays in them, but I can definitely see the value in getting a pickup and a pull-behind if you might be using your camper to live and work in. Linda is right, though, trailers are easier to steal than motorized RV's, so if you're keeping all of your worldly possessions in there, I would want to be parked someplace that is relatively secure.
If you do choose to stay with a motorized RV, I would recommend going with the 21 footer. From I driving standpoint, 21' is only marginally bigger than 18', but it gives you a lot more interior and/or storage space.